This invention relates to optical coupling arrangements and, more particularly, to optical coupling arrangements suitable for use in high bandwidth, free-space optical communications systems operating at terra-Hertz (THz) signaling rates.
Although the use of fiber optic networks is extensive and growing, laying fiber cable continues to be expensive; especially for the xe2x80x9clast milexe2x80x9d connection to a customer""s premises. An attractive alternative to cable may be provided by a free-space optical link if sufficient optical power, e.g., at least 100 mW and preferably 1 W or more, can be obtained using an xe2x80x9ceye-safexe2x80x9d wavelength in the spectral region between 1.4 and 2-xe2x96xa1m. However, at the present time, commercially-available free-space communications transceivers, which utilize modulated diode lasers as transmitters, offer a maximum transmitter modulation rate of only about 650 MHz, which is significantly slower than high-speed fiber optical network data rates of 10 to 40 GHz.
Achieving signaling rates above 650 MHz presents a twofold challenge. First, stray capacitance must be minimized which dictates that solid state detectors have a very small active area, e.g., an aperture of less than 25-30 xcexcm in diameter. This presents a significant challenge in receiver design since optical signals must be gathered, aligned with and coupled into a very small aperture of the solid state detector surface with minimal loss.
At the outset, it would seem to be quite straight forward to use a lens (or mirror) to focus a light beam down to any desired spot size. To minimize the size of the receiver it is desirable to use a lens with a short focal length. However, to gather the largest column of incident light, it is desirable to select a lens system with the largest possible entrance pupil. This dictates that a lens with a small xe2x80x9cfxe2x80x9d number (the ratio of focal length to diameter of the entrance pupil) be used. Achieving the small spot size desired (e.g., a diameter of 25-30 xe2x96xa1m), limited only by diffraction effects, also dictates that a small xe2x80x9cfxe2x80x9d number be used. Because lens aberrations, atmospheric turbulence, misalignment, and manufacturing errors all cause the focused spotsize to be much larger than the diffraction-limited size, use of a smaller xe2x80x9cfxe2x80x9d than that dictated solely by diffraction effects seems unavoidable. On the other hand, lenses with small xe2x80x9cfxe2x80x9d number are quite expensive and difficult to manufacture.
Heretofore some attempts have been made to use lens systems to address certain aspects of the coupling and alignment problems arising with laser usage. For example, U.S. Pat. No. 6,026,206 discloses an optical coupling arrangement that uses two lenses to compensate for the elliptical beam pattern produced by conventional 980 nm erbium-doped fiber pumped lasers. One lens is disposed at the output of the laser and a second, anamorphic (i.e., a radially non-symmetric) lens is interposed between the conventional lens and the endface of the optical fiber which is to receive the signal. The anamorphic lens converges the fast axis beam with the slow axis beam thereby providing an essentially circular beam. The conventional lens serves to reduce the numerical aperture required of the anamorphic lens.
Another approach is shown in U.S. Pat. No. 5,698,452 in which a trapezoidally tapered waveguide abuts the surface of the photonic detector. The waveguide has a large entrance pupil to gather light which it directs by internal reflection to an exit pupil integrated with the photonic detector.
While the above-mentioned patents addressed some of the coupling and alignment problems presented, they either do not address the desirability of avoiding the need for lenses with especially low xe2x80x9cfxe2x80x9d numbers or do not show how to provide the high degree of light beam concentration required to achieve terrahertz signaling speeds.
In accordance with an aspect of the present invention, a cone of incident light is gathered and concentrated to the size of the active area of a photonic detector aperture dimensioned to minimize capacitance effects commensurate with achieving the desired terra-Hertz (THz) signaling speed. The concentration of incident light is effected by the reflective surface contour of compound parabolic concentrator element having focii that define the perimeter of the detector aperture. The compound parabolic concentrator is a non-imaging element having a reflective surface whose contour may be described by rotating a parabolic arc about an axis oriented at an angle to the parabolic arc""s axis that is determined by the conical angle of the incident light desired to be gathered. The compound parabolic reflective surface is displaced from the rotational axis at its light gathering end by the radius of the desired entrance pupil and at its light concentration end by the radius of its exit pupil. Fabricating the concentrator element of a light transmitting material having an index of refraction, n, the maximum achievable concentration ratio Cmax is determined by the squares of the radii of the entrance and exit pupils and the half angle xcex8 of the maximum cone of incident light to be gathered by:                                           C            max                    =                                                    (                                  a                                      a                    xe2x80x2                                                  )                            2                        =                                          n                2                                                              sin                  2                                ⁢                                  θ                  max                                                                    ,                            (        1        )            
where, a and axe2x80x2 are the radii of the entrance and exit pupils.
In one embodiment, a refractive lens is employed at the input to the concentrator to focus incident light to as small a spot at the entrance aperture of the concentrator as may be determined by the combined effects of diffraction, lens aberrations, atmospheric turbulence, misalignment, and manufacturing errors. The concentrator element then further concentrates the light and directs it to its exit aperture. In a further embodiment, which facilitates flexible alignment with the photonic detector, the exit aperture of the concentrator may be joined to a finite length of coated, cladded, or un-coated fiber instead of being directly affixed to the detector. In a still further embodiment, the fiber connecting the concentrator to the detector may be arranged to provide pre-amplification ahead of the detector by employing erbium doped fiber amplification (EFDA) using forward and reverse pumping or other gain modes.